1) Field of the Invention
The present invention relates to an internal pressure measuring device for measuring an internal pressure in a living body using a catheter inserted into the living body from an electric signal output from a transducer connected to the proximal end of the catheter for converting the pressure into an electric signal.
2) Brief Description of the Prior Art
Examples of the above-described type of internal pressure measuring device include an invasive type blood pressure measuring device. When blood pressure measurements are conducted with this invasive blood pressure measuring device, zero adjustment is conducted prior to the measurements by opening the pressure-receiving surface of the transducer to the atmosphere in a state in which the atmosphere-opening point of the transducer is located at the same level or height as the pressure reference point, i.e., the heart (the tricuspid valve thereof), of the object to be measured.
However, this invasive type blood pressure measuring device has a vital drawback. That is, as the position of the object to be measured varies, the head of the blood in the catheter varies with respect to the transducer When the measured values are about several mmHg, as in the case of the central venous pressure, variations in the head of the blood may be about 0.73 mmHg/cm, which cannot be ignored. Hence, re-adjustment of the atmosphere-opening point of the transducer to the height of the pressure reference point is required for stably monitoring the blood pressure over a long period of time.
To obviate this problem, the present inventors disclosed in Japanese Patent Laid-Open No. 19132/1990 the internal pressure measuring device using a catheter with at least three lumens which are: the internal pressure measuring lumen whose distal end is open; the position correcting liquid-charged lumen whose distal end is closed and in which a liquid is charged; and the position correcting atmosphere-opened lumen whose distal end is closed, whose proximal end is made open to the atmosphere during the measurements, and having on the surface thereof which adjoins the distal end portion of the position correcting liquid-charged lumen a membrane which does not transmit liquid but transmits air. In this internal pressure measuring device, an internal pressure is detected from the signal output from the pressure measuring transducer mounted on the base and connected to the proximal end of the internal pressure measuring lumen. The internal pressure value is corrected using the head of the blood corresponding to the position of the membrane which is detected by the internal pressure measuring transducer connected to the proximal end of the liquid-charged lumen.
In the above internal pressure measuring device, since the distal end of the liquid-charged lumen is located at the same level as the pressure reference point, adjustment of the position of the pressure transducer, which would otherwise required when the measurement is started, is eliminated. Also, variations in the position which would occur thereafter can be automatically corrected. However, the above-described internal pressure measuring device has problems in that position measurement response is slow due to the use of the membrane and in that measurement errors may occur due to changes in the volume caused by changes in the temperature, e.g., in the body temperature, resulting from the difference in the coefficient of expansion between the liquid closed by the membrane and the liquid-charged lumen. The latter problem cannot be ignored particularly when a low pressure is measured by the device, as in the case of the central venous pressure.